Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $49.63   13.130%   $56.1464   $56.15   $56.15   $56.10  
  Purchase #2 $5.57   7.567%   $5.9915   $5.99   $6.00   $6.00  
  Purchase #3 $91.76   12.721%   $103.4328   $103.43   $103.45   $103.40  
  Purchase #4 $223.56   8.523%   $242.614   $242.61   $242.60   $242.60  
  Purchase #5 $40.96   13.858%   $46.6362   $46.64   $46.65   $46.60  
  Purchase #6 $134.46   7.774%   $144.9129   $144.91   $144.90   $144.90  
  Purchase #7 $250.63   5.268%   $263.8332   $263.83   $263.85   $263.80  
  7 purchase totals = $796.57       $863.5670   $863.56   $863.60   $863.40